Tuberoinfundibular neurons of the rat and monkey brain will be identified by retrograde accumulation of wheat germ agglutinin after infection or diffusion into the median eminence. Using immunohistochemical techniques, these neurons will be further characterized with respect to their content of specific hypophysiotropic substances including somatostatin, LHRH, ACTH, Beta-lipotropin, tyrosine hydroxylase, neurotensin, TRH, cholecystokinin-8, bombesin and methionine enkephalin. In addition, methods to simultaneously identify surface receptors on immunohistochemically defined tuberoinfundibular neurons will be sought using the histamine I receptor as the model. Studies will also be undertaken to further characterize a substance which is present in the rat tuberoinfundibular neuronal system that has human growth hormone-like immunoreactivity. These studies will entail determination of its molecular weight and amino acid sequence and addresses the possibility that it may be a precursor molecule to TRH.